Vehicle air-conditioner systems have been proposed that have a plurality of electric air blowers (e.g., two air blowers) and a motor driving device that drives each of the air blowers together. For instance, a conventional configuration of an electrical circuit for the motor driving device is shown in FIG. 3.
The motor driving device 10 includes inverter circuits 20, 30, an oscillation circuit 31, and field-effect transistors 32, 33. The inverter circuit 20 drives a three-phase brushless motor MO1 for the air blower based on a pulse signal P0, which is outputted from an electronic control unit 40 (ECU).
More specifically, a stator of the three-phase brushless motor MO1 has armature windings A, B, C that provide a rotor with a rotating magnetic field. The inverter circuit 20 increases a rotational speed of the three-phase brushless motor MO1 (i.e., the rotor) by applying a higher voltage to the armature windings A, B, C when the duty ratio of the pulse signal P0 increases. With reference to FIG. 3, the duty ratio is expressed as: TH/TH+TL.
The inverter circuit 30 drives a single-phase brushless motor MO2 for the air blower based on the pulse signal P0 outputted from the ECU 40. More specifically, the inverter circuit 30 outputs a pulse signal P1 to the field-effect transistor 32, and outputs a pulse signal P2 to the field-effect transistor 33.
The pulse signals P1, P2 have the same frequency of f1 and there is a 180° phase shift between these two signals as shown in FIGS. 4A and 4B. The inverter circuit 30 increases the corresponding duty ratio of each of the pulse signals P1, P2 according to an increase in the duty ratio of the pulse signal P0. Additionally, electric motors employed as the brushless motors MO1, MO2 are different from each other due to different volumes of required air to be supplied.
The oscillation circuit 31 outputs a pulse signal P3 that has a constant frequency of f2 (e.g., 20 KHz), which is greater than f1 as shown in FIG. 4C. The pulse signal P3 is output to a corresponding gate terminal of each of the field-effect transistors 32, 33.
Consequently, as shown in FIG. 4D, the field-effect transistor 32 switches on and off at the frequency of f2 during a period H1 based on the pulse signal P3 outputted from the oscillation circuit 31 and the pulse signal P1 from the inverter circuit 30.
As shown in FIG. 4E, the field-effect transistor 33 switches on and off at the frequency of f2 during a period H2 based on the pulse signal P3 outputted from the oscillation circuit 31 and the pulse signal P2 from the inverter circuit 30.
The single-phase brushless motor MO2 is connected to the field-effect transistors 32, 33 and a direct-current power supply Vcc. The stator of the single-phase brushless motor MO2 has armature windings A, B that provide the rotor with the rotating magnetic field.
Accompanying the switching of the field-effect transistor 32, an electric current from the direct-current power supply Vcc passes through the armature winding A. The electric current from the direct-current power supply Vcc passes through the armature winding B following the switching of the field-effect transistor 33.
Because of the 180° phase shift between the pulse signals P1, P2 as described above, the switching of the field-effect transistor 32 alternates with that of the field-effect transistor 33. Accordingly, energization of the armature winding A from the direct-current power supply Vcc alternates with that of the armature winding B.
The oscillation circuit 31 increases the duty ratio of the pulse signal P3 as the duty ratio of the pulse signal P0 outputted from the electronic control unit 40 increases. Furthermore, as mentioned above, the inverter circuit 30 increases the corresponding duty ratio of each of the pulse signals P1, P2 according to the increase in the duty ratio of the pulse signal P0. As a result, as the duty ratio of the pulse signal P0 increases, an average electric current that passes through the armature windings A, B from the direct-current power supply Vcc increases. Thus, the rotational speed of the single-phase brushless motor MO2 increases in proportion to the duty ratio of the pulse signal P0. Consequently, the rotational speeds of the brushless motors MO1, MO2 are controlled by means of the duty ratio of the pulse signal P0 from the electronic control unit 40 so that the brushless motors MO1, MO2 are driven together.
However, conventional systems such as the motor driving device 10 described above have certain problems. For instance, in motor driving device 10 described above, the oscillation circuit 31 that oscillates at the constant frequency of f2 is employed in controlling the rotational speed of the single-phase brushless motor MO2. The oscillation of the oscillation circuit 31 generates undesired radiation, which can adversely affect other devices, such as audio devices (e.g., radio), video devices (e.g., television), and the like.